1. Field of the Invention
This invention relates to a needle board for an apparatus for needling a non-woven fabric, comprising a carrier plate provided with an elastic layer, which is parallel to said plate and serves to retain needles, which are detachably held in through holes of the carrier plate and have needle butts, which are angled from the needle shanks and supported on the surface of the needle board, wherein the needle shanks are guided in the through holes of the carrier plate.
2. Description of the Prior Art
The needles of a needle board used to needle a non-woven fabric are subjected to wear and are also subjected to loads which may result in a damage to the needles. For this reason the needles must detachably be mounted in the needle board. It is known to provide a needle board consisting of a carrier plate, which is provided with an elastic surface layer on that side of the needle board which is remote from the non-woven fabric that is to be needled. As a result, the individual needles which extend in through bores through the carrier plate and through the elastic surface layer will be held against falling out during a manipulation of the needle board performed for a replacement of the needles even when the needles are only loosely held in the through bores of the carrier plate. But because the elastic surface layer must support the angled needle butts and for that purpose must have a certain hardness, the desired retention of the needles in the elastic surface layer will no longer be ensured after a few replacing operations. In order to avoid said disadvantages it is known form German Utility Model 81 04 031 to permit the angled needle butts to snap into dovetail grooves of the elastic surface layer. But even if the needle butts are additionally retained in that manner the required minimum hardness of the surface layer will involve a risk that a repeated replacement of needles will expand the dovetail grooves beyond the elastic limit. Besides, even that design cannot suppress a wobbling motion of the needles in the through bores of the carrier plate if a certain play of the needles in the through bores has resulted after a repeated replacement of needles.
It is known that the radial play can be avoided in that sleeves of plastic or spring steel are inserted in the through bores of the carrier plate but this will involve a considerable expenditure, which is usually not accepted, particularly because the sleeves may not be inserted into the through bores with the required care so that the needles may assume an incorrect orientation so that the risk of damage to the needles will considerably be increased.
In another known needle board known from British Patent Specification No. 1,157,772 the needles are not guided in through holes of a carrier plate but are guided in two elastic surface layers, which cover said carrier plate on both sides, so that inaccuracies in the drilling of the through bores in the carrier plate will be avoided and the needles can more easily be replaced. As the needles extend through the through holes in the carrier plate with a considerable radial clearance so that they are not guided by the surfaces defining such holes, the elastic surface layers must be relatively hard so that said layers can guide the needles although said guiding functions will not perfectly be performed even if the surface layers are relatively hard. On the other hand, the use of the harder layers will involve the disadvantages explained hereinbefore.